1. Field of the Invention
The present invention relates to a lateral photodetector, and a method for manufacturing the same.
2. Description of the Related Art
Lateral photodiodes, in which p-type and n-type regions are arranged parallel to the surface of the substrate, are known as described for example in Japanese Unexamined Patent Publication No. 5(1993)-175536. FIG. 8 illustrates a basic structure of a lateral photodiode. As illustrated, a semiconductor layer 2 is formed on a substrate 1, and a p-type region 3 and an n-type region 4 are formed parallel to the surface of the substrate 1 within the semiconductor layer 2. Electrodes 8 and 9 are connected to the p-type region 3 and n-type region 4 respectively, and an insulation layer 7 is formed to cover the semiconductor layer 2 from above. In a lateral photodiode having such structure, absorption of light and movement of carriers occur in an area of the semiconductor layer near the surface, so that it may realize a high speed operation compared to a well known vertical photodiode, in which p-type and n-type regions are stacked in the thickness direction of the substrate.
The demand for even higher speed for such lateral photodiodes, however, is growing. That is, in conventional lateral photodiodes, Si (silicon) forming p-type and n-type regions has low absorption rate for light with wavelengths in the long wavelength range of 650 nm and above, which is often the detection target range in general purpose applications, so that a considerable time is required for movement of carriers, which is one of the impediments to a high speed response.
As structures for realizing a high speed response in lateral photodetectors, the following are known. That is, a lateral photodetector having a deep trench structure, in which comb electrodes connected to the p-type and n-type regions are formed, is known as described, for example, in non-patent literature “A high-speed, high-sensitivity silicon lateral trench photodetector”, by M. Yang et al., IEEE Electron Device Letters, vol. 23, Issue 7, pp. 395-397, 2002 (non-patent document 1). Further, another type of lateral photodetector in which a high speed response is realized by isolating the active layer from the substrate using an electrical or a physical barrier layer is also known as described, for example, in non-patent literature “Design and implementation of high-speed planar Si photodiodes fabricated on SOI substrates” by C. Schow et al., IEEE Journal of Quantum Electronics, vol. 35, Issue 10, pp. 1478-1482, 1999 (non-patent reference document 2) and Japanese Unexamined Patent Publication No. 2004-200685.
In the mean time, a lateral photodetector that includes an active layer having a p-layer and an n-layer stacked in the thickness direction of the substrate to form a p-n junction is described in non-patent literature “Large Diameter, CMOS-Manufacturable Photodetectors for over 2 Gbps Polymer Optical Fiber Applications”, by W. P. Giziewicz et al., Proceedings “Integrated Photonics Research and Applications”, 2006 (non-patent document 3). In the structure, the areas of the active region, to which electrodes are connected, are separated from the carriers of the substrate by means of the p-n junction.
Further, a lateral MSM (Metal-Semiconductor-Metal) photodetector, which includes a substrate having thereon an active region, and a cathode and an anode arranged parallel to the surface of the substrate, each forming a Schottky metal contact with the active region, is also known as described, for example, in U.S. Patent Application Publication No. 20040119093. In this type of lateral photodetectors, even higher response speed is demanded.
The structure described in the non-patent document 1, however, requires an additional manufacturing process for providing the deep trench structure, which makes the photodetector costly to manufacture. The structure described in the non-patent document 2 requires a high resistance material for the active layer, which makes the photodetector difficult to fabricate. If a high resistance material is not used in the active layer, the whole area of the active layer does not become a depletion layer, so that the carriers need to move by diffusion and a sufficient speed is not obtained.
In the mean time, it is known that a frequency response is extremely slow for strong input light, and high speed response is not obtained in the structure described in the non-patent document 3.